Add a regression test for #4837 (#4847)

It was already fixed in master, we need to release 2.10.0.

Author: Pierre-Sassoulas

tests/functional/u/unused/unused_private_member.py

@@ -241,3 +241,25 @@ def __init__(self):
             print("Error")
             FalsePositive4681.__instance = False  # This should be fine
             FalsePositive4681.__should_cause_error = False  # [unused-private-member]
+
+
+class Pony:
+    """https://github.com/PyCQA/pylint/issues/4837"""
+    __defaults = {}
+    __defaults_set = False
+
+    def __init__(self, value):
+        self.value = value
+
+    def __init_defaults(self):  # [unused-private-member]
+        if not self.__defaults_set:
+            type(self).__defaults = { "fur": "pink" }
+            type(self).__defaults_set = True
+
+    def __get_fur_color(self):  # [unused-private-member]
+        color = lookup_attribute(self.__defaults, "fur")
+        return color
+
+
+def lookup_attribute(mapping, key):
+    return mapping[key]

tests/functional/u/unused/unused_private_member.txt

@@ -14,3 +14,5 @@ unused-private-member:212:8:Crash4755Context.__init__:Unused private member `Cra
 unused-private-member:229:4:FalsePositive4681:Unused private member `FalsePositive4681.__should_cause_error`:HIGH
 unused-private-member:239:12:FalsePositive4681.__init__:Unused private member `FalsePositive4681.__should_cause_error`:HIGH
 unused-private-member:243:12:FalsePositive4681.__init__:Unused private member `FalsePositive4681.__should_cause_error`:HIGH
+unused-private-member:254:4:Pony.__init_defaults:Unused private member `Pony.__init_defaults(self)`:HIGH
+unused-private-member:259:4:Pony.__get_fur_color:Unused private member `Pony.__get_fur_color(self)`:HIGH